Combination of top nozzle and guide thimble for nuclear fuel assembly

ABSTRACT

A joint structure between a top nozzle and a guide thimble of a nuclear fuel assembly and, more particularly, a structure for joining an inner-extension tube, the top nozzle and the guide thimble. When an inner-extension tube head, which is provided as a means for facilitating removal of the top nozzle of the nuclear fuel assembly from the guide thimble, is removed from an inner-extension tube body to separate the top nozzle from the nuclear fuel assembly, the inner-extension tube body is prevented from undesirably rotating, so that the guide thimble and the inner-extension tube body can maintain the joined state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-part Application of U.S. application Ser. No.12/187,455, filed on Aug. 7, 2008, which claims the benefit of priorityfrom Korean Patent Application No. 10-2007-0086066, filed on Aug. 27,2007, the disclosures of both of which are expressly incorporated byreference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to joint structures between topnozzles and guide thimbles of nuclear fuel assemblies and, moreparticularly, to a joint structure between a top nozzle and a guidethimble which is configured such that an inner-extension tube isprevented from undesirably rotating when the top nozzle is separatedfrom the nuclear fuel assembly.

2. Description of the Related Art

A nuclear reactor refers to a device that is designed to exertartificial control over the chain reaction of the nuclear fission offissile materials, thereby achieving a variety of purposes such as thegeneration of heat, the production of radioisotopes and plutonium, theformation of radiation fields, or the like.

Generally, enriched uranium that is obtained by raising the ratio ofuranium-235 to a range between 2% and 5% is used in a light waternuclear reactor. The uranium is molded into a cylindrical pellet thatweighs 5 g and processed into nuclear fuel that is used in a nuclearreactor. Numerous pellets are embedded into a cladding tube made ofZircaloy which is in a vacuum state. Thereafter, a spring and helium gasare put into the tube, and then a top end closure stopper is weldedthereon, thereby making a fuel rod. A plurality of fuel rods constitutesa nuclear fuel assembly and is burned in a nuclear reactor by nuclearreaction.

FIG. 1 is a schematic view showing a general nuclear fuel assembly.

Referring to FIG. 1, the nuclear fuel assembly includes a skeleton and aplurality of fuel rods 1. The skeleton includes a top nozzle 4, a bottomnozzle 5, a plurality of spacer grids 2, a plurality of guide thimbles 3and a instrument tube 6. The fuel rods 1 are inserted longitudinallyinto an organized array by the spacer grids 2 in such a manner as to besupported by means of springs (not shown) and dimples (not shown) whichare formed in the spacer grids 2. In order to prevent the formation ofscratches on the fuel rods 1 and damage to the springs upon assemblingthe nuclear fuel assembly, lacquer is applied to the surfaces of thefuel rods 1 before the fuel rods 1 are inserted longitudinally into theskeleton of the nuclear fuel assembly. Subsequently, the top and bottomnozzles are secured to the opposite ends of the nuclear fuel assembly,thereby finishing the procedure of assembly of the nuclear fuelassembly. After the lacquer is removed, the following items of theassembled nuclear fuel assembly are tested: the distance between thefuel rods, distortion, dimensions including the length, etc., thuscompleting the process of manufacturing the nuclear fuel assembly.

As shown in FIG. 2, the top nozzle 4 includes a hold-down plate 42,hold-down springs 43, inner-extension tubes 45, outer guide posts 44,and a flow plate 41.

Referring to FIGS. 1 and 2, the inner-extension tubes 45 of the topnozzle 4 are connected to the respective guide thimbles 3 so that thenuclear fuel assembly can be firmly fixed in the reactor and thestructural stability of the nuclear fuel can be ensured during theburn-up of the nuclear fuel.

The top nozzle 4 and the guide thimbles 3 are joined to each other insuch a way as to be removably connected to each other, thereby ensuringa path along which the fuel rods 1 can be drawn out when disassemblingthe top nozzle 4. Disassembly of the top nozzle 4 from the guidethimbles 3 is carried out in a storage tank. A worker must remotelyperform the disassembly work to minimize the harm caused by radiation.Accordingly, the joint structure between the top nozzle 4 and the guidethimbles 3 must be designed such that assembly or disassembly betweenthem can be conducted remotely.

FIGS. 2 and 3 illustrate a typical method of connecting the guidethimbles 3 with the top nozzle 4. Referring to the drawings, the methodof joining the guide thimbles 3 and the top nozzle 4 will be described.As shown in FIG. 2, an external thread is formed on a lower end 451 ofeach inner-extension tube 45. As shown in FIG. 3, an internal thread isformed on an inner surface of a threaded portion 31 of each guidethimble 3. The top nozzle 4 and the guide thimbles 3 are joined witheach other by thread-coupling. An external thread is formed on a lowerend of each outer guide post 44. The outer guide posts 44 are threadedlycoupled to the flow plate 41. The threaded lower end of each outer guidepost 44 is partially welded to the flow plate 41 to prevent the outerguide post 44 from rotating. Furthermore, in order to prevent eachinner-extension tube 45 from becoming loose, a head of theinner-extension tube 45 is partially crimped in a radial direction insuch a way as to be put in contact with the outer guide post 44.Moreover, the inner-extension tube 45 can be separated from the outerguide post 44 only when torque of more than a specific strength isapplied to the head.

However, in the state where the inner-extension tube 45 is joined withthe outer guide post 44, when the inner-extension tube 45 of the topnozzle 4 is rotated to be separated from the outer guide post 44, sincethe distance between an outer surface of the inner-extension tube 45 andan inner surface of the outer guide post 44 is too short, it isdifficult to rotate the inner-extension tube 45 along the threads ifconcentricity is not congruous or if foreign substances have gottenstuck between the outer face and the inner face. That is, due tofrictional heat generated by the contact surface, the inner-extensiontubes 45 and the outer guide posts 44, which are made of stainlesssteel, are fused together by a cold welding effect, and hence, looseningdoes not occur.

To solve the above-mentioned problems, there have been disclosed U.S.Pat. No. 4,702,883 entitled “Reconstitutable fuel assembly havingremovable upper stops on guide thimbles”, and U.S. Pat. No. 4,687,630entitled “Top nozzle and guide thimble joint structure in a nuclear fuelassembly”.

In the prior arts, heads of outer guide posts are removed without anyinner-extension tube, and processed to have threads so as to minimizethe contact surface when the outer guide posts are removed.

Furthermore, each guide thimble is threadedly coupled to a threadedportion of a lower end of the corresponding outer guide post. Thus, twothreaded coupling portions are respectively formed on upper and lowerends of each outer guide post.

Accordingly, when the head of each outer guide post is rotated to removethe top nozzle, since the outer guide post and the head thereof arethreadedly-coupled with each other, the thread-coupling between theouter guide post and the guide thimble may become loosened.

Hence, in order to prevent the lower end of the outer guide post frombecoming loosened, the outer guide post is equipped with a wedge device;however, this has the problem of the assembling and disassemblingprocesses being complicated.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a joint structure between a top nozzle and aguide thimble which is configured such that when an inner-extension tubehead that has been threadedly coupled to an inner-extension tube body isremoved from the inner-extension tube body to disassemble the topnozzle, the inner-extension tube body can be prevented from beingundesirably removed from the guide thimble.

In order to accomplish the above object, the present invention providesa joint structure between a guide thimble and a top nozzle of a nuclearfuel assembly, the guide thimble being coupled to a spacer grid of thenuclear fuel assembly, the top nozzle including: a flow plate locatedabove the guide thimbles, with a coupling through hole formed throughthe flow plate; an outer guide post coupled at a lower end thereof tothe coupling through hole of the flow plate; an inner-extension tubedisposed in the outer guide post in such a way that a lower end of theinner-extension tube passes through the coupling through hole of theflow plate; and an inner-extension tube head coupled both to an upperend of the inner-extension tube and to an upper end of the outer guidepost, the inner-extension tube head connecting the inner-extension tubeand the outer guide post to each other, wherein rotation-preventingmeans is provided in at least one of a junction between theinner-extension tube and the coupling through hole of the flow plate anda junction among the inner-extension tube head, the inner-extension tubebody and the outer guide post, the rotation-preventing means preventingthe inner-extension tube body from rotating when the inner-extensiontube head is rotated.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic view of a conventional nuclear fuel assembly;

FIG. 2 is a partial sectional view of a conventional top nozzle;

FIG. 3 is a sectional view of a conventional guide thimble;

FIG. 4 is a perspective sectional view showing the joining of a topnozzle to a guide thimble according to the present invention;

FIG. 5 is a perspective sectional view of an outer guide post accordingto the present invention;

FIG. 6 is a perspective view of an inner-extension tube according to thepresent invention;

FIG. 7 is a perspective sectional view of a flow plate according to thepresent invention;

FIG. 8 is a perspective view showing an inner-extension tube accordingto a first embodiment of the present invention;

FIG. 9 is a perspective view showing an inner-extension tube accordingto a second embodiment of the present invention;

FIG. 10A is a perspective sectional view showing an embodiment of theflow plate according to the present invention;

FIG. 10B is a bottom view showing a joint structure between the flowplate and the inner-extension tube of the first embodiment of thepresent invention;

FIG. 10C is a bottom view showing a joint structure between the flowplate and the inner-extension tube of the second embodiment of thepresent invention;

FIG. 11 is a perspective view showing an inner-extension tube bodyaccording to a third embodiment of the present invention;

FIG. 12 is a perspective sectional view of a flow plate according to thethird embodiment of the present invention;

FIG. 13 is a perspective sectional view of an outer guide post accordingto a fourth embodiment of the present invention;

FIG. 14 is a perspective view of an inner-extension tube body accordingto the fourth embodiment of the present invention; and

FIG. 15 is a perspective sectional view of an inner-extension tube headaccording to the fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is provided to achieve the above-mentioned object.

The present invention is characterized by the constructions of an outerguide post 120, an inner-extension tube 150, a flow plate 160 and aguide thimble 3.

(Therefore, a hold-down plate and a hold-down spring which are elementsof a top nozzle but are not directly related to the present inventionwill not be explained herein.)

Hereinafter, a joint structure among the above elements will bedescribed in detail. FIG. 4 illustrates the joint structure.

Referring to FIG. 5, the shape of the outer guide post 120 is that of ahollow cylinder that has open upper and lower ends. An external thread102 is formed on a predetermined portion of an outer surface of a lowerend of the outer guide post 120 and is used to couple the outer guidepost 120 to the flow plate 160.

The diameter of a predetermined portion of an upper end of the outerguide post 120 larger than those of other portions of the outer guidepost 120. Thus, an annular retaining part 104 is formed by thisdifference in diameter so that the outer guide post 120 can be joinedwith a hold-down plate (not shown) by means of the annular retainingpart 104.

As shown in FIG. 6, the inner-extension tube 150 includes aninner-extension tube body 130 and an inner-extension tube head 140.

The shape of the inner-extension tube body 130 is that of a hollowcylinder that has open upper and lower ends in the same manner as thatof the outer guide post 120. Furthermore, the inner-extension tube body130 is longer than the outer guide post 120 and is disposed in the outerguide post 120 in such a way that an upper end of the inner-extensiontube body 130 is level with that of the outer guide post 120 while itslower end protrudes outwards from that of the outer guide post 120.

An external thread 132 is formed on a predetermined portion of the lowerend of the inner-extension tube body 130, in other words, on the portionof the inner-extension tube body 130 that protrudes outwards from theouter guide post 120. The external thread 132 is used to couple theinner-extension tube body 130 to the guide thimble 3 which will beexplained in detail later herein.

The inner-extension tube head 140 is coupled to the upper end of theinner-extension tube body 130. The shape of the inner-extension tubehead 140 is that of a cork stopper of which upper and lower ends are thesame in shape but are different in diameter. Further, theinner-extension tube head 140 has a hollow structure which is open onupper and lower ends thereof. An internal thread 142 is formed on apredetermined portion of an inner surface of the lower end of theinner-extension tube head 140 so that it is threaded over acircumferential outer surface of an upper end 134 of the inner-extensiontube body 130.

Here, an annular retaining part 144 is formed by a difference indiameter between the upper and lower ends of the inner-extension tubehead 140. The maximum diameter of the annular retaining part 144 isequal to or larger than the diameter of the upper end of the outer guidepost 120 so that the inner-extension tube head 140 can be placed on theupper end of the outer guide post 120 rather than being insertedthereinto. Furthermore, the inner-extension tube head 140 has a thinfilm structure such that it is crimped into a depression of the outerguide post 120, thus preventing the inner-extension tube head 140 frombecoming loose.

A rotation-preventing surface 136 is formed on the lower portion of theinner-extension tube body 130.

The rotation-preventing surface 136 is disposed above the externalthread 132 that is formed on the lower end of the inner-extension tubebody 130. The rotation-preventing surface 136 is a planar surface formedby cutting out a portion of an annular flange provided above theexternal thread 132. The rotation-preventing surface 136 functions toprevent the inner-extension tube body 130 from rotating when theinner-extension tube head 140 is separated from the inner-extension tubebody 130.

As shown in FIG. 7, a coupling through hole 162 is formed at apredetermined position in the flow plate 160 so that the outer guidepost 120 is coupled to the inner-extension tube 150 via the screwcoupling the upper peripheral surface on the hole 162.

An female screw is formed on a circumferential inner surface of thecoupling through hole 162. The diameter of the annular retaining part isthe same as the outer diameter of the outer guide post 120. A lowersurface of the annular retaining part is put into close contact with anupper surface of the annular flange provided on the inner-extension tubebody 130.

The diameter of an upper end of the coupling through hole 162 is thesame as the outer diameter of the outer guide post 120. An internalthread is formed on a circumferential inner surface of the upper end ofthe coupling through hole 162 so that the lower end of the outer guidepost 120 is threaded into the coupling through hole 162.

A rotation-preventing portion 166 is formed on a lower end of the innersurface of the coupling through hole 162. The rotation-preventingportion 166 is disposed at a position corresponding to therotation-preventing surface 136 of the inner-extension tube body 130 andhas a polygonal shape corresponding to the shape of therotation-preventing surface 136.

A threaded portion 31 is provided on an upper end of the guide thimble 3(refer to FIG. 3). The shape of the threaded portion 31 of the guidethimble 3 is that of a hollow cylinder that has an open upper end. Thediameter of the threaded portion 31 is the same as that of theinner-extension tube 150. An internal thread is formed on the innersurface of the threaded portion 31.

The lower end of the inner-extension tube body 130 is threadedly coupledto the threaded portion 31 of the guide thimble 3.

A process of disassembling the top nozzle from the guide thimble will beexplained with reference to FIG. 6.

First, the inner-extension tube heads 140 are rotated with respect tothe corresponding inner-extension tube bodies 130 and removed therefrom.Thereafter, the top nozzle that includes the outer guide posts 120, theflow plate 160, hold-down springs (not shown) and the hold-down plate(not shown) is separated from the nuclear fuel assembly.

When each inner-extension tube head 140 is separated from thecorresponding inner-extension tube body 130, the thread-coupling betweenthe inner-extension tube body 130 and the threaded portion 31 of thecorresponding guide thimble 3 may become loose. However, by virtue ofthe rotation-preventing surface 136 formed on the inner-extension tubebody 130 and the rotation-preventing portion 166 provided on the flowplate 160, the undesirable separation of the inner-extension tube body130 from the threaded portion 31 of the guide thimble 3 can beprevented.

FIGS. 8 through 10 illustrate first and second embodiments of therotation-preventing surface of the inner-extension tube body and therotation-preventing portion of the flow plate.

Unlike the inner-extension tube body 130 described above, several planarsurfaces may be formed on the annular flange of the inner-extension tubebody 130 to provide a plurality of rotation-preventing surfaces 135.

In detail, as shown in FIG. 8, the rotation-preventing surfaces 135 areformed on the annular flange at positions spaced apart from each otherat angular intervals of 90°. Planar cut portions 133 are formed onopposite sides of each rotation-preventing surface 135.

The planar cut portions 133 formed between the rotation-preventingsurfaces 135 function to prevent interference between fuel rods and theannular flange of the inner-extension tube body when the fuel rods,which are disposed adjacent to the guide thimble in spacer grids of thenuclear fuel assembly, are inserted longitudinally into or drawn outfrom the nuclear fuel assembly.

The second embodiment of the inner-extension tube body 130 isillustrated in FIG. 9.

As shown in FIG. 9, rotation-preventing surfaces 138 are formed on anannular flange of the inner-extension tube body 130 at positions spacedapart from each other at angular intervals of 90°. Recesses 139 areformed in the annular flange on opposite sides of eachrotation-preventing surface 138 at positions spaced apart from eachother by a predetermined distance. The recesses 139 conduct the samerole as that of the planar cut portion 133 described in the firstembodiment.

The flow plate 160 which is coupled to the inner-extension tube body 130of the first or second embodiment may be used in such a way that flowplates that correspond to the respective flanges of the inner-extensiontube bodies of the first and second embodiments are separately provided.However, as shown in FIG. 10A, the lower end of the coupling throughhole of the flow plate 160 preferably has a shape corresponding to theannular flange of the inner-extension tube body 130 that is providedonly with the rotation-preventing surfaces 135 without any planar cutportion 133. In this case, as shown in FIG. 10B, when theinner-extension tube body 130 is coupled to the flow plate 160, therotation-preventing surfaces 135 are put into close contact with theflow plate 160, but portions of the annular flange other than therotation-preventing surfaces 135 are spaced apart from the flow plate160 rather than making contact with it.

Furthermore, the flow plate 160 that has the shape of FIG. 10A can becoupled not only to the inner-extension tube body of the firstembodiment but also to that of the second embodiment. As shown in FIG.10C, the general shape of the annular flange of the inner-extension tubebody of the second embodiment is the same as that of the annular flangeof the inner-extension tube body of the first embodiment, except for adifference in shape between the planar cut portions 133 and the recesses139. Therefore, the flow plate of FIG. 10A can be used for theinner-extension tube body of the second embodiment.

FIGS. 11 and 12 illustrate a third embodiment with regard to therotation-preventing surface of the inner-extension tube body and therotation-preventing portion of the flow plate.

In this embodiment, a plurality of rotation-preventing protrusions 137are provided on the inner-extension tube body 130 and arranged in thecircumferential direction at the same positions as those of therotation-preventing surfaces 136 of the inner-extension tube body 130described above.

Rotation-preventing recesses 167 that have shapes corresponding to therotation-preventing protrusions 137 are formed in the flow plate 160 atthe same positions as those of the rotation-preventing portions 166 ofthe flow plate 160 described above.

The joint structure between the inner-extension tube body and theinner-extension tube head of the inner-extension tube or between theinner-extension tube body and the outer guide post may be embodied byforce-fitting rather than thread-coupling (in a fourth embodiment).

Referring to FIG. 13, an outer guide post 220 is shaped like a hollowcylinder that has upper and lower ends that are open. An external thread202 is formed on a predetermined portion of an outer surface of a lowerend of the outer guide post 220. The external thread 202 is used to jointhe outer guide post 220 with a flow plate 160 which will be explainedlater herein.

The diameter of a predetermined portion of an upper end of the outerguide post 220 is larger than at other portions of the outer guide post220. Thus, an annular retaining part 204 is formed by this difference indiameter so that the outer guide post 220 can be joined with a hold-downplate (not shown) by means of the annular retaining part 204.

A coupling groove 206 is formed in an inner surface of the outer guidepost 220. The coupling groove 206 is used to join the outer guide post220 with an inner-extension tube which will be explained later herein.

The inner-extension tube includes an inner-extension tube body 230 andan inner-extension tube head 240.

As shown in FIG. 14, the shape of the inner-extension tube body 230 isthat of a hollow cylinder that has open upper and lower ends in the samemanner as that of the outer guide post 220. Furthermore, theinner-extension tube body 230 is longer than the outer guide post 220and is disposed in the outer guide post 220 in such a way that an upperend of the inner-extension tube body 230 is level with that of the outerguide post 220 while its lower end protrudes outwards from that of theouter guide post 220.

An external thread 232 is formed on a predetermined portion of the lowerend of the inner-extension tube body 230, in other words, on the portionof the inner-extension tube body 230 that protrudes outwards from theouter guide post 2220. The external thread 232 is used to join theinner-extension tube body 230 with the guide thimble 3 which will beexplained in detail later herein.

Furthermore, a circumferential coupling protrusion 234 is provided onthe inner-extension tube body 230. The coupling protrusion 234 isdisposed on a circumferential outer surface of the inner-extension tubebody 230 at a position corresponding to the coupling groove 206 formedin the inner surface of the outer guide post 220 so that when theinner-extension tube body 230 is inserted into the outer guide post 220,they are coupled to each other by the coupling groove 206 and thecoupling protrusion 234.

A plurality of longitudinal slits 236 of a predetermined length areformed in the upper end of the inner-extension tube body 230. Thelongitudinal slits 236 make it possible for the upper end of theinner-extension tube body 230 to move elastically so that when theinner-extension tube body 230 is inserted into the outer guide post 220,the coupling protrusion 234 can be easily hooked into the couplinggroove 206.

Further, a coupling groove 238 is formed in an inner surface of theinner-extension tube body 230. The coupling groove 238 is used to jointhe inner-extension tube body 230 with an inner-extension tube head 240,and it will be explained later herein.

A rotation-preventing surface 239 is formed on the lower portion of theinner-extension tube body 230.

The rotation-preventing surface 239 is disposed above the externalthread 232 that is formed on the lower end of the inner-extension tubebody 230. The rotation-preventing surface 239 is a planar surface formedby cutting out a portion of an annular flange provided above theexternal thread 232. The rotation-preventing surface 239 functions toprevent the inner-extension tube body 230 from rotating when theinner-extension tube head 240 is separated from the inner-extension tubebody 230.

As shown in FIG. 15, the inner-extension tube head 240 is coupled to theupper end of the inner-extension tube body 230 and has the shape of acork stopper whose upper and lower ends are the same in shape butdifferent in area. The inner-extension tube head 240 has a hollowstructure which is open on upper and lower ends thereof.

A coupling protrusion 242 extends in a circumferential direction and isprovided on a circumferential outer surface of a lower portion of theinner-extension tube head 240. The coupling protrusion 242 is disposedat a position that corresponds to the position of the coupling groove238 of the inner-extension tube body 230 when the inner-extension tubehead 240 is joined with the inner-extension tube body 230.

A fitting protrusion 244 which extends in a circumferential direction isprovided on the circumferential outer surface of the lower portion ofthe inner-extension tube head 240. The fitting protrusion 244 functionsto apply pressure to the inner surface of the inner-extension tube body230 outwards when the inner-extension tube 250 is coupled to the outerguide post 220, thereby strengthening the coupling between them.

A plurality of through holes 246 are formed in the circumferentialsurface of the lower end of the inner-extension tube head 240. Thethrough holes 246 allow a tool or the like to be inserted thereinto tofacilitate removal of the inner-extension tube head when disassembling.A taper 248 is formed in the lower end of the inner-extension tube head240, thus making it easy to insert the inner-extension tube head 240into the inner-extension tube body 230 or remove it therefrom.

The joining between the top nozzle and the guide thimble using theforce-fitting structure seldom causes the inner-extension tube body tobe rotated. Therefore, this joint structure can be used regardless ofthe presence of the rotation-preventing surface

As described above, in the present invention, a rotation-preventingsurface is formed on an inner-extension tube, and a rotation-preventingportion is formed on a flow plate. Thus, when a top nozzle is separatedfrom a guide thimble, the inner-extension tube is prevented fromundesirably rotating, thereby preventing the inner-extension tube frombecoming loosened from the guide thimble. Therefore, a separaterotation-preventing member is not required. Furthermore, because thearea of the contact portion between elements that rotate can beminimized, the assembly or disassembly of the top nozzle can befacilitated, thus reducing the time required to assemble or disassemblethe structure.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A combination of a guide thimble and a top nozzlefor a nuclear fuel assembly, the combination comprising: a guidethimble, and a top nozzle coupled to the guide thimble, the top nozzlecomprising: a flow plate located above the guide thimble, with acoupling through hole formed through the flow plate; an outer guide postcoupled at a lower end thereof to the coupling through hole of the flowplate; an inner-extension tube disposed in the outer guide post in sucha way that a lower end of the inner-extension tube passes through thecoupling through hole of the flow plate; and an inner-extension tubehead coupled to an upper end of the inner-extension tube and to an upperend of the outer guide post, wherein: the inner-extension tube includesa rim disposed within the coupling through hole of the flow plate; therim has a circumferential outer surface which includes a first regionand a second region; a circumferential inner surface of the couplingthrough hole of the flow plate includes a first part disposed at aposition corresponding to the first region of the rim of theinner-extension tube and a second part; and the first region and thefirst part, collectively, prevent the inner-extension tube from rotatingwhen the inner-extension tube head is rotated.
 2. The combination as setforth in claim 1, wherein the first region is formed as a planarsurface; and the first part is formed as a planar surface.